This invention relates to apparatus for measuring the time-correlation between two signals. More particularly, it is concerned with apparatus for measuring the time-correlation between received impulse signals and synchronizing pulses in the presence of noise.
In certain types of communication systems designed to insure the confidentiality of transmitted messages, received signals are correlated with a synchronizing signal at the receiver to determine if the received signals include valid message data. The synchronizing signal may be obtained in any of various ways and made available at the receiver. The degree of correlation between the received input signal and the synchronizing signal is a measure of the authenticity of the message data being received.
Various techniques and apparatus are known for measuring the degree of correlation between received input signals and a synchronizing signal. Systems have been devised for determining the degree of correlation in the presence of noise which tends to obscure the correlation information in the received input signal. In apparatus heretofore available for measuring correlation in the presence of noise, correlation data for each measurement is collected over a period of time. If an excessive noise level occurs at any time during a period, all correlation data accumulated during the period is dropped. The loss of correlation data is particularly high when the data rate is low in comparison with the rate at which noise measurements are being made.